Man. I'm really torn.I'm new to the world of photography and I'm looking to add equipment to the camera bag.I've got the Canon Rebel XT (8mp).The 75-300 f/4.0-5.6 IS Canon lensThe lens that came with the camera.And the 580EX Speedlite.

I was looking at 16-35 f/2.8L USM but couldn't bring myself to part with the cash, especially since it would just be "replacing" one of my lenses and no adding to my camer's abilities. So I started looking at Macro lenses (and maybe a macro flash).

My problem is this - I don't entirely know what specifications makes one lens better/different from another.I know the f/4.0-5.6 references the maximum aperture settings (based on the zoom seetting on the lens)....And of course I know that the aperture affects the light and depth of field.So can I assume that the lower the f/ number the larger that lens aperture capablities are... and that this is a "better lens" than those with a higher F/ number (smaller aperture) because more light can get in (although it reduces depth of field)?Am I over simplifying this?

One more thing...I thought that you could use the focal length to determine say the zoom on a lens.I thought the calculation for a 35mm film camera with a 300mm lends would have a calculation something like 300mm / 35mm = 8.6 x zoom. (focal length / sensor size)... sounds reasonable... I think.

Now - when it comes to digital - the sensor is smaller so the calculation changes. I heard the 1.6x factor all the time. This kinda of makes sense. Looking at the Rebel XT sensor of 22.2 x 14.8 you can make the calculation of 35mm / 22.2mm = 1.6. I just love it when numbers make sense don't you? Things like the Fibonacci Sequence and FHI (1.618) just make my heart a flutter ;-) .... But I digress...

Now. If you put a 35mm lens on a 35mm camera - the image should be 1:1 - no wide or zoom right?

Now if you put that same lens on the rebel you should have the following calculation right :35mm / 22.2mm = 1.6 times zoom right? To get a 1:1 you'd have to have a lens with a focal length of around 22 right?But when I put the lens that came with the camera on (I believe a 18-55) 22 seems like wide angle (when looking throught the lens). 55 seems to be 1:1 which is reverse to what I was expecting. To get a zoom factor of 1 the formula changes to (35 / sensor size) X 35mm (35 being the size that lenses are engineered to put with) = 56 (focal length) ie 1:1. (I'm rounding numbers)Have I lost anyone yet? I may have lost myself there.I admit there seems to be a pattern here with the calculations but I can't figure out the logic behind them.

So if I put a 300mm lens on my Digital Rebel what is the zoom factor?By the calculation I would have thought focal length / sensor size (300mm / 22m) the zoom is 13.5. Is that correct? If it is, then why is it that the 55mm setting on the other lens looks like a 1x zoom setting but calculates out to be a 2.5x zoom?

Oh...Sorry I forgot to mention.My whole reason for going through this exercise of calculations is not only to figure out what the specs for the lens mean, but also to use that knowledge to help me pick out the next purchase to add to the camera bag.